In apparent motion, static stimuli presented successively in shifted locations produce a subjective percept of continuous motion. It has been shown that reducing stimulus exposure (or on-time) increases the perceived velocity of the apparent motion. Surprisingly, little investigation has followed up on the illusion's discovery. The objective of this study was to delineate the boundary conditions of the on-time effect in order to clarify its underlying mechanism. Subjects viewed multi-item apparent motion displays. At a random point during the trial, on-time duration either increased or decreased, while objective velocity remained unchanged. Subjects were asked to judge whether the motion became slower or faster. The on-time effect was replicated in four experiments: it was observed in both fast and slow apparent motion displays; it was not modulated by stimulus luminance, thus precluding an energy-summation account of the illusion; it generalized to a time-perception paradigm; finally, it was found to be specific to apparent motion: with veridical motion, the effect reversed. We suggest that perceived velocity in motion is derived from a weighted average of exposure times to objective motion signals and to static motion signals.